You can use an Adobe InDesign newsletter as a Mailchimp template by following these steps. First, export your InDesign design as an HTML file. You can do this by selecting "File" then "Export" and choosing HTML format. Next, open Mailchimp and go to the "Templates" section. Click "Create Template" and select "Code Your Own." Choose "Paste in Code" and paste the HTML from your InDesign export. You may need to tweak the code for proper formatting in Mailchimp. Once you're happy with how it looks, save the template for future use. Test it by sending a test email to ensure it appears correctly.